Let go
by Subtle Shenanigans
Summary: "He was tired . . . He was just, so tired. So he breathed deeply, and sighed once. And then he let go." In which Gaster let's himself die, and Sans turns into a Boss Monster. And the author isn't good at word play. BossMonster!AU with BossMonster!Sans and brief implication of BossMonster!Gaster. UNBETA'D. R and R please. (And yes there will be sequel(s).)


**A/N: So I sort of got this idea while reading** _ **Threads**_ **by FlamingFlyingFoxOfDoom, at least in the concept of Sans as a Boss Monster.**

 **This is also heavily influenced by** _ **CORE**_ **by Trefoil-Underscore (which is a** _ **fantastic**_ **story), in terms of Gaster being a Boss Monster and basically having taken Sans under his wing. Also the idea of Boss Monsters "passing" down to those adopted.**

 **Seraphim Sans from** _ **The Thought**_ **may have influenced me a bit too.**

 **Now, I don't know if this is already an AU or not or really close to one, but this idea is going to be called the BossMonster!AU, with BossMonster!Sans (or Boss!Sans as BM wouldn't work for obvious reasons.)**

 **This plays off my personal favourite Gaster-Skelebros idea (though the Boss Monster part is new), which I'll give a quick overview of:**

 **Basically, Sans is the older brother and Papyrus was very little (maybe a little kid, like four or five, so Sans was maybe Seven or Eight) when Gaster found them and took them in. He didn't exactly adopt them yet. But he did care for them (even if he was absent and at work a lot which Sans constantly argued with him about.)**

 **Eventually as Sans got older, he showed an interest in science and began to work in the lab with Gaster. They grew closer, and Gaster began to view him as a son and best friend. (Not that he didn't see Papyrus as a son, just that he was closer with Sans. And Sans saw himself as closer with Papyrus than Gaster.)**

 **Now, when a Boss Monster has a kid with another monster, whether biologically or magically from the parents, they will always be a Boss Monster because that magic gets passed down. A Boss Monster who does not have children must eventually find a charge to pass that Magic down to. (It's not that it's necessarily disastrous if they don't, but it's a deeply instinct-driven thing. Boss Monsters can fall into a deep depression and loneliness or develop volatile, usually anger-based, emotional issues if this magic is not shared. It's unknown how long before this effect takes place or why it occurs at all.)**

 **So Gaster decided to pass that down to Sans.**

 **But, as it is with the case of passing down to adopted children, they can only pass it on to one monster, and it's not effective until the parent Boss Monster actually dies. When they die, the child monster will change slightly physically and experience change in HP, ATK, and DEF. Changes vary depending on the monster passing and the monster receiving.**

 **So basically this is about Gaster finally fading from the Void and Sans tuning into a Boss Monster.**

 **(I may do a background one-shot about this another time.) Oh! And Frisk isn't in this timeline, at least for the time being. It's just our Underground friends. Flowey is somewhere.**

 **Design will be drawn for BossMonster!Sans and posted on Instagram ( stmb (underscore) ff).**

 **DISCLAIMER:** **Undertale belongs to Toby Fox, or The Annoying Dog. Who** _ **really**_ **knows.**

* * *

 _Let go_

* * *

He was so tired.

He had held himself together for _so long._ Trying, _trying_ to cling to this not-quite existence, in hopes of returning.

But it's not as if anyone remembered him. He didn't exist. Not anymore. The only one who remembered him didn't fully, and the other barely knew him as wisps of a faded figure in a forgotten dream.

 _. . . and he was so tired_.

He had nothing to lose.

And did he really have anything to gain?

They were doing well, from what he saw when he dragged himself from _nothing_ to see the _something_. They were young adults, and doing so well. He was proud.

( _But he wasn't.)_

He would miss them.

( _Ha! But he couldn't._ )

He loved them and hoped that somehow, that little bit, would stay with them.

( _But how could something that didn't exist have feelings? There could be nothing. There_ _ **was**_ _nothing. He was nothing.)_

 _(But then how did he feel and think and know?)_

 _(It was funny, really, how he had become an anomaly of sorts.)_

He was just, so _tired_.

So he breathed deeply, and sighed once.

And then he let go.

* * *

 _It was like the sound of a thousand stained-glass windows shattering, and no other sound except that._

 _And then there was truly nothing._

* * *

It was a typical Thursday; Papyrus and Undyne had done a rigorous training session, then met up with Sans after his hot-dog shift, and they had strolled around for an hour before making their way to Undyne's house. Alphys was to meet them there with a few choices of movies (but not anime - that was on Saturdays) that she and Sans would sort through while Papyrus and Undyne . . . _enthusiastically_ made dinner.

Alphys had already ordered the pizzas.

Alphys was already there, bubbly greeting them and tripping over her words excitedly as she clutched a decent stack of VHSes. Most of them didn't have covers but Sans did see at least one _Classic Doctor Who_ and something called . . . _The Goonies_?

Undyne had already shoved Papyrus in through the door, Alphys squawking at her to be careful, when Sans felt an odd twinge in his chest.

He paused, brow-bone (magically) scrunching up in confusion, before he simply shrugged, following the rest inside.

He had already piped up, joining in the banter as they all continued to the kitchen, when it happened again. Worse.

He gasped, clutching at his bony chest with a mittened hand, his other pressed against the wall as he doubled up in pain because _dangit that hurt!_

"Sans?!"

Papyrus had already rushed over, supporting his brother who only groaned in response. Alphys had basically dropped the VHSes on the counter and hurried over, Undyne at her heels.

"What's wrong w-with him?!"

"I don't know! Sans? Sans?! Are you okay?! What's happening?!"

"Sans, you better answer you punk! Sans!"

Sans' eye lights briefly flickered to Payrus' face, but before he could say anything, they sputtered and went out.

Sans fell limp in his grasp, the only sign he was alive was the slight trembling that softly shook his frame. He felt warm.

Papyrus looked to Alphys imploringly and with worry, to which she just tugged on his arm to make him stand, stuttering, "L-let's get him to th-the lab!"

Paps nodded, carrying his brother delicately in his arms, and rushed out after Alphys, Undyne right behind them as they made their way to Hotland.

* * *

They laid him on a quick fold-out bed in Alphys' "lab", the little lizard quickly swiping everything off of her desk with a frantic fury none knew she could ever possess. She quickly got to setting up equipment, like a magic monitor and Soul scanner (she almost set up a heart monitor too but then remembered it would be useless here). It was a good thing she had a doctorate in biology; at least she knew how to use the equipment and have it on hand.

Papyrus stood by, watching with a wide, nervous gaze and wringing his gloved hands worriedly. Undyne stood next to him, shoulders touching in an attempt at comfort, with her arms crossed.

Sans wasn't looking so good; once in awhile his eyelights would flicker back, especially pale, like a sick monster rolling their eyes during a fever dream. Sweat beaded on his skull. He was still trembling - though now it was more of a shuddering - soft groans emanating from him with the occasional whimper.

"Is this some sort of Skeleton sickness?" Undyne asked her friend.

Papyrus answer was enough to scare her, even without the choked tone. "No."

Alphys was fiddling with the wires so she could attach them to Sans, when she heard him gasp sharply. She turned her gaze fully just in time to see him jerk, then fall back as his soul rose from his chest.

It didn't go high, and was still lying 'flat'. The soft-white, upside-down heart pulsed once, then sank back into Sans' body, which relaxed back with it. He gave an almost inaudible sigh, falling into a relaxed sleep.

They all stared at him for a few moments.

Undyne was the first to break the silence.

"Well that was weird." She snorted.

"What…" Papyrus looked at Alphys with the closest thing to "puppy-dog eyes" he could manage (which was actually pretty good, which made sense since he hung around the Snowdin Dogs so much). "What just happened."

To say Alphys was thunderstruck would be the grossest understatement. She knew what had happened. She had _heard_ of it - knew of at least two monsters it had happened two, one of which was still potentially alive today (the other long, long gone.) But what she didn't get was _how._

"Alphys?" Undyne asked softly. She was still next to Papyrus, the latter having grabbed her arm to anchor himself. They both looked to the Royal Scientist, waiting for whatever answers she had.

She put her face in her scaly claws for a moment, taking a few, deep, shuddering breaths, then straightened up.

"Okay, okay," she muttered to herself, not meeting their eyes. She clenched her fists. "Okay," she said quietly, then looked at them, straightening her glasses. "So, uhm, b-best as I can tell, Sans is … Sans is, erm…"

"Sans is what?!" Paps and Undyne ask in exasperated unison.

" _Sans is turning into a Boss Monster!_ " Alphys blurted, face red and startled.

" _What?!_ "

Alphys had gone back to attaching the wires to Sans, the magic monitor still recalibrating and the Soul scanner just starting it's cheery chirp as she went on.

"F-from what I can tell it's a passing - meaning some Boss Monster h-had adopted Sans as their son. I don't know if-f it's because he's older t-than you, or if they just ch-chose him rather than you." She looked over her shoulder to Papyrus for confirmation.

Papyrus nodded, still confused. "Yes Sans is older. But who would … ?"

Alphys had gone back to fiddling with the dials on the magic monitor. "I d-don't know. The only Boss Monsters I know of-f are King Asgore and Lady Toriel - b-but they already had a ch-child. They _c-couldn't_ give a passing; t-that's not how it w-works."

"Paps?" Undyne nudged him.

Papyrus kept staring at the floor, face drawn in tight confusion. ". . . Sans pretty much raised me, but I think once, maybe a long time ago? Or not? There was a time we lived with someone else? But I can't remember him?"

Undyne nudged Papyrus, grinning. "You said 'him'! So there must have been someone else!"

Papyrus have a small smile. Then he saw Sans completely unmoving, and that smile fell into something sad. ". . . So what do we do now?"

Alphys had finished fiddling with the dials, taking a step back and putting her claws on her hips as she examined her handiwork.

"Now," she said without turning. "Now, we wait."

* * *

It had been three days since Sans had fallen, and in all that time he hadn't woken up once.

There were a few notable changes - he seemed to have grown a little, though they couldn't tell how much yet, and the edge of his smile had developed fangs. His slippers and gloves which had been removed during his first day of blessed unconsciousness revealed that his finger tips and toe tips had changed to have claw-like ends. Alphys was sure there were other changes they couldn't see, and if Sans had been awake she's sure he would be screaming as his magic restitched his body into something new.

The magic monitor did display an influx of magic, which would faintly burst at intervals, but she had no idea what his stats would increase to. Some adopted Boss Monsters jumped a few points, some a lot, and she had even heard that there was a case once where one had doubled.

( _She knew the old queen had been passed, as had Asgore's great-great-grandfather. She wished she could ask them about this. But one was dead and the other gone._ )

The Soul scanner showed no change from the normal pulse a Soul should have.

* * *

His eyelights flared as he finally woke up.

He groaned, curling up tighter on his side. He was so _tired_. But he also felt like he couldn't sleep. He scrunched his eye sockets closed for a moment, the with a huffed, weary sigh, sat himself up.

" _Ughhhh . . ._ " He groaned, popping his vertebrates. He felt funny, like his body wasn't the right size anymore. He rubbed his head, briefly registering that _his gloves were gone_ , and that his fingers scratched a little rougher than he was used to. He sat slumped for a moment, then yawned loudly.

Huh.

Now _that_ felt strange.

He just sat there, pleasantly in a mind fog. He watched blearily when Alphys entered, huffing a soft laugh when she dropped her papers with a squeak.

" _Sans! You're awake!"_

"Heya Al," he greeted sleepily.

Heh. Now _that_ didn't sound quite right.

She noted some of the things different about him, but dismissed them as she ran off, saying, "I-I'll be right back! S-stay right there!"

He shrugged, unconcerned, as she left.

Alphys was back in a jiffy with Undyne and Papyrus, the former shoving past and lifting Sans in a _bone-crushing_ (heh) hug.

"Oh Sans you are okay! How do you feel? Does anything hurt? Do you feel ill? Should Alphys-"

"Woah, woah; slow down bro. Why don't you finish setting me down and start again, _slower_ this time, 'kay Paps?" He patted his brother's hands where he gripped Sans' shoulders.

Papyrus blinked rapidly at him. " _Sans_. I already _did_ set you down."

Sans blinked once, slowly, then glanced at where his feet were already planted on the cold, tile floor. "Huh."

During their conversation Papyrus had indeed already set him down, and it was then they all noticed.

Sans wasn't really that much shorter than Papyrus, originally. He was maybe a little around a foot shorter. And whereas he had been maybe one or two inches taller than Alphys, he was now probably five or six inches taller.

He looked back up at everyone, fingers twitching. "Huh," he said again, slowly stuffing his hands back into his jacket pockets, careful of the claws.

It was awkwardly silent, with unspoken questions and answers they didn't quite know how to get across.

"Your voice sounds funny," Undyne supplied, the brow above her remaining eye quirking.

That set them going. Papyrus elbowed her, shouting, " _RUDE!_ " and Alphys squeaked, hiding behind her claws in embarrassment. And Sans?

He laughed.

It started out as a huff, then quickly escalated into full-blown chortling that could rival the door-Lady's howls. The earlier tension and awkwardness began to melt away as Papyrus and Undyne argued, Alphys timidly interjecting but for the most part hiding.

(To be fair, Sans' voice did sound strange. The pitch had fallen a little lower, and was almost _softer_ in a sense. But not like when a person was trying to speak quietly kind of soft. No, because dust can hit the floor loudly but the sound is still a soft _shfffth_.

The pitch also wasn't as low as his voice when he let it echo darkly with emotion and finality.)

They did finally turn the conversation to what had happened. Sans had been surprised to learn he was now a Boss Monster, head tilting slightly and eye sockets barely widening. But he took it in stride like he did most everything; cracking a pun and acting nonchalant like _his body hadn't just restructured itself._

His eyelights flickered as he took in Papyrus' offended groaning at the ludicrous wordplay, and Undyne's aggressively loud vocalization of her displeasure. His eyes flicked to Alphys, and his grin quirked up, not quite as intimidating with the fangs as Alphys thought it would be. She smiled back.

( _They all had noticed his eyes; it was hard not to. The lights were bright but not in a concentrated way, the edges expanded with a soft dim almost in a facsimile of iris around pupil._

 _It was interesting, really._ )

The only other noticeable, physical change was that he didn't look as squat and rounded. His shirt and jacket were still loose on him (perhaps due to magic) but more so baggy as to give him an almost gangly look (but nowhere near as lanky as Papyrus, of course.)

"Sans," Alphys eventually dressed him through the chatter. "I'm going to need to check your stats."

Everything fell quiet as Sans stiffened and Papyrus fidgeted. After a moment, Sans loosened, giving a stiff nod. "Ok."

Papyrus looked at him. "Sans…"

Sans tilted his head, smiling. "It's 'kay, Paps. I'll honestly be surprised if it's changed. It's not like I need-" he winked his left eyesocket. "- to be _defensive._ "

"OH MY GOD SANS."

" _What?_ Are my puns _attacking_ you?"

"SANS THESE ARE THE WORST ONES YET!"

"Are you saying you _hope_ I'll stop?"

" _SANS! Nyyeeehh!"_

"Okay you two," Undyne stepped between the grimacing and grinning brothers. She glared at Sans, her yellow eye brooking no nonsense. "Let's get this on with."

"Y-yes please. Just one m-minute," Alphys said, cleaning her glasses with the edge of her sleeve.

She blew air on them, then held them up to the light, squinting. Giving a satisfied nod, she set them on her snout. "Okay. Let's-do this."

She stood across from Sans, who had his feet placed in a battle stance. She didn't enter a conflict, but _CHECKED_ him with his consent.

It was an open _CHECK_ , so they all mentally saw the floating stats suspended in their "sight."

 _SANS_

 _HP 2/2_

 _ATK 2_

 _DEF 2_

It was deathly silent for a moment, long enough for the _CHECK_ to have worn off. Sans shuffled for a moment. "Huh. My stats doubled."

" _What do you mean they doubled, punk?!"_

" _You only had o-one HP?!"_

Papyrus, though, was ecstatic, sweeping his brother up in another hug and spinning around the room with him, laughing joyfully all the while. "Nyeheheheheheh!"

Sans couldn't help but join in with his own deeper chortles. "Heheheheheheheh…."

All the while Undyne was screeching frustratedly in the background and Alphys squealing in terror, while the two brothers spun and laughed, with tears edging in their eyesockets, and Sans-

His Soul hadn't felt so light in so long.

And he didn't ever want to let go.


End file.
